According to the currently applicable EU limiting values, particle filters are not yet required in gasoline-powered internal combustion engines. The exhaust gas aftertreatment of the pollutant components, such as non-combusted hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), is performed in the case of homogeneous concepts via a three-way catalytic converter. In the case of lean-burn combustion systems, a storage catalytic converter for the NOx is connected downstream. However, there is also a three-way catalytic converter situated close to the engine here, which fulfills the three-way function in homogeneous operation and is required for the oxidation of CO and HC in lean operation. For homogeneous concepts, a main catalytic converter, which may also be situated structurally separated, for example, in the subfloor of the vehicle, is often connected downstream from the primary catalytic converter.
Future stricter exhaust gas limiting values with respect to particle emissions as required, for example, from 2014 by the EU limiting values according to the exhaust gas regulation EU6, require that gasoline-powered internal combustion engines will also have to be equipped with a particle filter. The exhaust gas flow is conducted in this case, identically to diesel-engine use, through the particle filter (GPF=gasoline particle filter) situated in the exhaust gas system, which separates the solid particles located in the exhaust gas and retains them in the filter substrate. The soot mass deposited in the filter results in steady clogging of the filter and thus an increase of the exhaust gas counter pressure, however, which has a negative effect on the engine performance and the fuel consumption.
The particle filter effect is based on a porous ceramic substrate having alternately closed channels, which the exhaust gas is forced to flow through. The soot particles are deposited in this case on the surface and in the walls (depth filtration) of the porous substrate. These particle filters clog with soot during operation and must therefore be regenerated at certain time intervals. The maximum soot load of the particle filter is decisively a function of the substrate material of the filter, for example, the porosity, the cell density, and the geometry of the channels, and in particular the melting temperature and the thermal capacity. This is also true for catalytically coated particle filter substrates which, in addition to a three-way catalytic converter effect, may also retain soot particles, and are therefore also referred to as four-way catalytic converters (FWC).
Conventional regeneration strategies are based on special injection profiles and air flow rates, so that an elevated temperature is achieved in the exhaust gas channel of the internal combustion engine and, in the event of oxygen excess, the oxidation of the soot may occur. Manifold measures are used for this purpose, because the required high exhaust gas temperatures of 600° C. to 650° C. are only achieved close to full load in normal operation.
In a multicylinder internal combustion engine, the cylinders are frequently situated in two cylinder banks. The air required for the combustion is supplied to all cylinders via a shared intake manifold. An air flow meter may be provided therein, using which the air mass suctioned in via the intake manifold is measurable. On the outlet side, separate exhaust gas channels are connected to the two cylinder banks, which are also referred to as exhaust gas banks. An exhaust gas sensor, which is provided for measuring the composition of the exhaust gas, is assigned to each of these exhaust gas channels. In a gasoline engine, both exhaust gas sensors are typically implemented as lambda sensors. The exhaust gas aftertreatment is typically performed by catalytic converters situated close to the engine in the separate exhaust gas channels or with the aid of a catalytic converter situated in a shared exhaust gas channel, the separate exhaust gas channels typically first being joined together into the shared exhaust gas system at the subfloor of the vehicle. These are referred to as so-called Y-systems.
The fuel quantities to be injected into the two cylinder banks are each separately calculated by a control unit as a function of the output signals of the lambda sensors situated in the exhaust gas channels of the two cylinder banks, a control factor, which influences the injection of fuel into the respective associated cylinder bank, being calculated in each case as a function of the output signals of the two lambda sensors. This control factor is typically generated with the aid of a so-called lambda controller, a separate lambda controller being assigned to each of the two cylinder banks.
A method for lambda modulation is discussed in DE 10 2006 003487 A1, in an internal combustion engine having a first group of cylinders whose exhaust gases are guided through a first exhaust gas channel and whose fuel/air mixture ratio is set via a fuel metering system, activated via a control unit, by a first control loop for setting a first lambda value based on the signal of a first exhaust gas sensor, which is situated upstream from a catalytic converter of the exhaust gas channel in the flow direction of the exhaust gas, and having at least one further group of cylinders, whose exhaust gases are guided through a further exhaust gas channel and whose fuel/air mixture is set via a fuel metering system by a further control loop for setting a further lambda value based on the signal of a further exhaust gas sensor, which is situated upstream from the catalytic converter or a further separate catalytic converter of the exhaust gas channel in the flow direction of the exhaust gas, the lambda modulation of the lambda values for the first exhaust gas channel and that of the further exhaust gas channel being synchronized. Using this method, a variation of the engine torque is reduced and therefore the driving comfort is improved during operation of an internal combustion engine having multiple exhaust gas banks. Furthermore, the catalytic converter diagnosis may therefore be improved in the case of multiple exhaust gas banks, which are joined together upstream from a shared catalytic converter (Y-systems).
In systems having an NOx storage catalytic converter (NSC), it is necessary to desulfurize the NSC at regular intervals, because it is clogged with SOx due to the sulfur content in the gasoline and therefore its NOx storage capacity is reduced. Therefore, for the desulfurization of the NSC, it is necessary to generate high exhaust gas temperatures and to introduce rich gas into the catalytic converter. In the case of a Y-system, this may be achieved by a so-called lambda split method, as is also understood to be used, for example, in DE 10 2006 003487 A1. One exhaust gas bank is operated rich, and the other slightly lean, so that in total, after the junction, a slightly rich exhaust gas results. In this way, non-combusted or partially combusted reactive components (HC and CO) and oxygen meet in the NSC. This results in the exothermic reaction and therefore in the temperature increase in the catalytic converter. Because the exhaust gas is slightly rich on average at the same time, the accumulated SOx is released.
Regenerating a particle filter in gasoline-powered engines (GPF) or also a coated particle filter (four-way catalytic converter or FWC) also requires an oxygen excess in addition to a high exhaust gas temperature. Therefore, during the regeneration, it is necessary to deviate from the typical λ=1 operating strategy in gasoline-powered internal combustion engines and to use a leaner lambda value. However, this is equivalent to an NOx breakdown, because the three-way catalytic converter or also the coated particle filter cannot convert NOx in lean operation.